Owing its inspiration and title to 'On the Origin of Species', James W. Valentine's ambitious book synthesizes and applies the vast treasury of phyla theory and research collected in the century and a half since Darwin's time. By investigating the origins of life's diversity, Valentine unlocks the mystery of the origin of phyla. One of the twentieth century's most distinguished paleobiologists, Valentine here integrates data from molecular genetics, evolutionary developmental biology, embryology, comparative morphology, and paleontology into an analysis of interest to scholars from any of these fields. He begins by examining the sorts of evidence that can be gleaned from fossils, molecules, and morphology, then reviews and compares the basic morphology and development of animal phyla, emphasizing the important design elements found in the body plans of both living and extinct phyla. Finally, Valentine undertakes the monumental task of developing models to explain the origin and early diversification of animal phyla, as well as their later evolutionary patterns.